Basic Instrument Concepts

The primary goal for the SORCE mission is to accurately measure the solar spectral irradiance at ultraviolet, visible, and near-infrared wavelengths. The mission consists of four separate optical channels: three spectrometers and one photometer.

A generic spectrometer uses an entrance slit and a dispersing element, followed by an exit slit, and finally a detector to record an isolated wavelength of the solar spectrum. A basic photometer has an aperture, a filter to isolate a fixed range of wavelengths, and a detector. Both measurement approaches require knowledge of the aperture area, the throughput of the instrument, the spectral bandpass (exit slit width for the spectrometer and effective filter bandwidth for the photometer), and the sensitivity of the detector in order to convert the raw signals into irradiances.

Data Provided by Instrument Overivew

SORCE provides precise daily measurements of the TSI with the TIM instrument, as well as the Solar Spectral Irradiance (SSI) at wavelengths extending from the far ultraviolet to the near infrared with the SOLSTICE, SIM, and XPS instruments. TIM monitors changes in incident sunlight to the Earth’s atmosphere via an ambient temperature active cavity radiometer. Using a new development that incorporates state-of-the-art technologies in its Electrical Substitution Radiometers (ESRs) while taking full advantage of new materials and modern electronics, the TIM measures TSI to an absolute accuracy of 100 parts per million (ppm), where 1 ppm = 0.0001%, with a precision and long-term relative accuracy of 10 ppm per year. The SOLSTICE instrument package is a derivative of the UARS SOLSTICE G- and F-channels; and in combination, they measure the solar UV irradiance from 115 to 320 nm with a spectral resolution of 0.1 nm. The SOLSTICE instrumentation on SORCE constitutes a continuation of the SOLSTICE solar-stellar comparison technique, as first demonstrated on the UARS SOLSTICE mission, launched in 1991. The two ultraviolet channels on SORCE are similar to the UARS SOLSTICE, in that they have the unique ability to observe bright stars using the same optics and detectors employed for the solar observations. In this way the stars become calibration standards against which the Sun is measured. SORCE also includes the newly developed SIM instrument to incorporate an entirely different technique using a self-calibrating optical system and an absolute ESR detector. The SIM uses a prism as the single optical element and an ESR as the primary detector. This instrument has a measurement requirement of 0.03% absolute accuracy and long-term relative precision of 0.01% per year. The XPS instrument measures the very energetic EUV and soft x-ray flux, where the solar variability exceeds a factor or two. Its precision and relative accuracy requirements are therefore on the order of ±10%.

Instrument Module (IM)

The five instruments, TIM, SIM, SOLSTICE A and B, and XPS, are mounted and precisely aligned on an optical bench. Also mounted to the bench are the sun sensors and the star trackers. This assembly, including electronic boxes and associated cable harness, is referred to as the Instrument Module (IM). The IM is mounted to the spacecraft.